I know a german guy, who bred a female papuan at 5 years old. The male was around the same age.
I think the most important issue regarding breeding them, is to find a pair that actually like eachother. Otherwise the smaller one, usually the male, could end up as a meal. They are highly cannibalistic for a python.
I have 2,1 myself. A pair around 2,2 meters and a 3 meter male. I just lost my 3 meter female to unknown reason. Maybe endo parasites. I was a week away from putting them together for breeding

Jonas